Gears and efficiency/speed For Christmas me and my brother got an AR Drone, the propellers are on a large gear, which is rotated by a small gear that is connected to the motor.
This means the small gear (and therefore the motor) must rotate many times in order to rotate the propeller just once.
Surely swapping the gears, so that the propellers are on a small gear, which is rotated by the large gear that is connected to the motor, would mean the propeller would spin faster (given that the motor is operating at the same speed as before), or that the motor could operate at a slower speed (and therefore use less energy) and maintain the same speed of the propeller as before?
Also what is the advantage of using any gears at all? Why not lose the gears, and get rid of the friction between the gears and between the gear and what holding them, and the added weight?
 A: Every motor has a torque vs. speed curve, and the product of those is power, and it has a speed at which it generates maximum power.
Every propeller generates thrust roughly proportional to its speed squared, or its power cubed.
So for a given power, it has a particular speed at which it uses that much power.
What you want to do in an airplane is find the propeller speed that absorbs the best power output of the motor.
On a typical small airplane, each propeller blade, of length about 3 feet, can absorb about 100 horsepower and generate about 200 lb of thrust at about 2400 rpm.
On the F4U Corsair, a WW2 fighter plane, the propeller was about twice that radius, so it had to turn at about 1/2 that speed. It had 4 blades, and each blade absorbed about 500 horsepower. Since the engine turned at about 2400 rpm, it had to have a 2-1 reduction gear to drive the propeller.
So it's a matter of matching the engine to the propeller.
Of course, you can throttle back the engine, in which case it uses less power by transmitting less power to the propeller, giving less thrust.
A: I believe that in simple terms the efficiency limit of a specific propellor goes down the closer it gets to the speed of sound. 
